Very pleased with my Ashley Hinton vent grills. it certainly encourages a better air flow into the car than the standard fine mesh that came with the car. I guess no grill at all would be even better for air flow but the vent void and it’s drain would be susceptible to blockages from leaves etc.
These grills from Ashley Hinton are a good compromise …. https://www.mg-cars.org.uk/ah/index.htm
MGB tips mods and maintenance 